1. Field of the Invention
The invention relates to methods and apparatus for treating surfaces of articles, such as semiconductor wafers, using combinations of inorganic acid and ozone.
2. Description of Related Art
Semiconductor wafers undergo a variety of wet processing stages during manufacture of integrated circuits, one of which is removal of photoresist from the wafer. When the photoresist is stripped by a wet process, among the techniques used for that stripping is a SOM (sulfuric acid ozone mixture) process. These processes involve dissolving ozone in sulfuric acid so that the ozone reacts with the sulfuric acid to form dipersulfuric acid or peroxydisulfuric acid (H2S2O8), as shown by the following equation:2HSO4−+O3<===>O2+H2O+S2O82−
Ozone that does not react with sulfuric acid can also dissolve as such into the sulfuric acid solution, and thus serve as an oxidizing agent for the material to be stripped.
SOM processes typically dissolve ozone into hot sulfuric acid (110 to 150° C.) before the mixture is dispensed onto a semiconductor wafer for a certain amount of time.
The present inventors have identified a number of disadvantages of the conventional SOM processes, including that the ozone has a significantly lower solubility in sulfuric acid at such high temperatures and second also a very short half life (the half-life of ozone in sulfuric acid at 150° C. has been found to be less than 10 seconds). Consequently, only small amounts of ozone and/or reactive peroxydisulfate ion reach the wafer and thus photoresist strip rates are relatively low, leading to long process times, low throughput and with this high cost-of-ownership.
In addition a continuous dispensing of the mixture is required for the whole process time in order to ensure a continuous supply of ozone and its reaction products to the wafer surface. For long process times this also results in a high chemical cost in terms of sulfuric acid, despite the ability to generate ozone in situ.
Still further, if only one chemical supply system is being used, which serves more than one process chamber, the distances for the chemical distribution lines are typically different to the individual process chambers, leading to a chamber-to-chamber variability in the actual ozone concentration delivered to each wafer.
Previous attempts to improve SOM processes have included dispensing sulfuric acid at a temperature in the range of about 25-150° C. onto the wafer and then providing ozone into the environment around the wafer (U.S. Pat. No. 6,869,487). In this way the ozone has to diffuse through the sulfuric acid layer to reach the photoresist to be decomposed. The diffusion process is however a rather slow process even when only a thin liquid layer is being created, and so most of the active decomposition products from ozone (active radicals with a very short lifetime) cannot reach the photo resist in sufficiently large quantities, leading to a non-optimum strip rate.
It has also been proposed to pressurize the heated liquid and then inject gaseous ozone to increase the concentration of the active species in the liquid (EP 1 100 630 B1, U.S. Pat. No. 5,971,368, U.S. Pat. No. 6,488,271). However, as soon as the pressure on the oversaturated ozonated liquid is reduced, during dispensing of the liquid onto the wafer, bubbles will form as the ozone comes out of solution, causing significant loss in the ozone concentration. Another limitation of this technique is that a pressurized chemical mixing system is required, which adds significantly to cost and raises safety concerns.